In the field of non-destructive hardness testing a large variety of tests are being performed using impact devices. An original concept was described in U.S. Pat. No. 4,034,603.
To determine the hardness of the sample to be tested, the rebound energy is assessed in some fashion; either as an absolute value or in a ratiometric way by relating it to the inbound energy.
Typically these devices consist of a tubular housing and a cylindrical or bullet-shaped impact body that can move linearly inside the housing. To perform a test, the impact body impacts on the surface to be tested with a certain energy. The majority of the impact devices use a spring to supply the impact energy.
Attention is now drawn to the two most common mechanisms.
1) In a first class of devices, the handle or the body of the device is operated to charge the spring to a maximum pressure, upon which a trigger releases the impact body automatically. After the impact action, the device returns to its “relaxed” state by means of a second spring. This mode of operation is considered the classic mode, since we find it, among others, in tools such as spring-activated centrepunches or devices to set nails or staples. Similar mechanisms are also common to weapons.
2) In a second class of devices, the spring is charged to maximum spring pressure and the device is held in the armed state. A trigger is released by a separate operation, e.g. by depressing a button. This is the concept described in e.g. U.S. Pat. No. 4,034,603.
A brief analysis of the two modes shows their drawbacks with regard to the scope of the device which is subject of the invention.
In case 1) the operator is physically stressed when charging the loading spring (which can store a considerable energy). This leads to tremor and slippage, in other words to unreliable results.
In case 2) the above problem is alleviated—the operator can trigger the device in a relaxed fashion—however, at the expense of an additional operation and a separate mechanical trigger. In cases where the testing device is actuated by a robotic arm, an additional trigger means a more complicated, less agile system.
FIG. 1 shows a typical impact device used for the testing of metal hardness that operates according to case 2) described above.
This type of impact device has become the industry standard (ASTM report).
Let us analyze the charging and trigger features of the classic device, since these will be superceeded by the present invention.
To “arm” the device, the impact spring is loaded by manually pushing the loading tube 1 towards the sample 2. This operation will move the catch chuck 3 by means of the tubular carrier 4 and, when in the fully compressed position, catch the impact body 5 by its anchor pin. The loading spring 6 will subsequently retract the assembly to its home position, thereby arming the impact spring 7. Actuating an external trigger button 8 will, by means of the push rod 9 and its conical tip 10, open the catch chuck 3 and release the impact body 5.
The signal processing and indicating circuitry 11 is either located in a separate unit or mounted directly onto the impact device. The circuitry must be powered in order to detect the voltage induced in the pick-up coil 12 and indicate a hardness reading.